Fix GRUB 2 locale

The current GRUB 2 installation code has a couple problems:

* It copies `/boot/grub/locale/en@quot.mo` to `/boot/grub/locale/%s.mo.gz`
  * The gz file is not gzip compressed
  * This will make GRUB 2 always in English
  * There's no need to copy anything. If you look at `/usr/sbin/grub-install`, it contains:

```sh
# Copy gettext files
mkdir -p "${grubdir}"/locale/
for dir in "${localedir}"/*; do
    if test -f "$dir/LC_MESSAGES/grub.mo"; then
        cp -f "$dir/LC_MESSAGES/grub.mo" "${grubdir}/locale/${dir##*/}.mo"
    fi
done
```

All the supported GRUB 2 languages are already copied to /boot/grub/locale/

* `grub-install` is run instead of `LANG=something grub-install`, so GRUB always ends up in English.

This pull request fixes both issues :)